The current GSM/3G and WLAN technologies constitute two separate radio network technologies. They operate at different frequencies and make use of separate designated protocols. Currently standardization work is ongoing on integrating data and other services of WLAN and GSM/3G in the forums of 3GPP, 3GPP2 and IEEE.
Six scenarios dealing with various degrees of integration have been sketched in the third generation partnership project, 3GPP. Scenarios 1, 2 and 3 have been defined so far. However, the work for scenario 6, i.e. WLAN GSM/3G integration of telephony (Circuit Switched voice services) has not yet started.
A desirable concept known as “always best connected” aims to ensure that the best and cheapest service connection is rendered available to the user at all times and performing handover in such a way that the user hardly notices the underlying access technology.
Currently, dual mode mobile phones integrating different kinds of radio access techniques, such as WLAN and GSM/3G, providing prior art access on the choice of the user by means of one apparatus have been rendered available on the marketplace. However, Cellular systems and WLAN systems are currently not interoperable on the network side.
A further problem for the integration of WLAN and GSM/3G services is the pico-cell character of the WLAN radio network. The WLAN radio network consists of a flat structure of pico-cells, not at all suitable for typical mobile user behavior. A telephony call would be difficult to maintain in a WLAN network when a user moves with high speed as he is used to in a cellular network.
Prior art document GB2336069 shows a dual use mobile station (10, 21, 41) having a GSM interface (451) and a local area interface, such as a RS-232 interface, W-LAN, USB or an IR interface (52) for connecting to a personal base unit (PBU) which comprises a (virtual) base transceiver station (BTS) emulator or virtual base station, which may be implemented by means of a personal computer (PC). The PBU has a LAN interface for connecting to an Interworking Unit (IWU). The interworking unit (IWU) is having on one hand connectivity for the PBU and a GMS base station transceiver and on the other hand connectivity to a Home Location Register (HLR), a Mobile Station Switching Centre (MSC) and an IP Plane. This allows cellular users to utilize private intranets to carry the cellular services within the intranet coverage area. Access to public GSM network is offered by introducing a transparent location management method, which allows terminals connected to the intranet to be reached from the GSM network in a normal manner. The Interworking Unit comprises among others an Intranet Location Register (ILR). The function of the Intranet Location Register is to store mobility management information and call statistics of the subscribers configured into the Wireless Intranet Office system. Roaming of visitors is controlled by the mobile switching centre. For visitors, only temporary information will be stored into the Intranet Location Register. When a mobile station is connected to the PBU, for example over the RS-232 cable or RF interface, information such as voice data, Fax SMS (Short Message Service) is transmitted over the local area network. The information and GSM layer 3 (i.e. Mobility management messaging, paging, page response, routing area updates etc.) signaling messages are re-directed to the RF interface branch. When the mobile station and the PBU are linked, the “field strength” of the virtual BTS will be (rendered) greater than that of other BTS's in the GSM network. Consequently, handover is made to the virtual BTS.
Prior art document U.S. Pat No. 6,119,003 shows a multimode cellular telephone capable of bi-directionally exchanging control messages and status messages with primary and secondary system controllers, such as a of a D-AMPS (Digital-Advanced Mobile Phone Service) and an AMPS network, for automatically switching the state of the terminal between primary and secondary systems. If the terminal is using its preferred network, then it stays connected to the primary network until the network is no longer available. If the latter applies the terminal begins scanning for the second network in an ordered priority list.
Prior art document US2004/0105434 shows a WLAN base station emulating the functionality of a BTS (Base Transceiver Station) in terms of GSM or CDMA (Code Division Multiplex Access) protocols. D3 bridges the gap between in-building WLAN network environment and the external cellular environment to ensure that a data link exists between the wide area network across the WLAN air interface provided a WLAN access point.
Once an interworking node (Ethercell) is aware that a dual mode cellular phone has been authenticated, it will assign a service point Identification value to the dual-mode cellular phone. Such assignment will be used for future communications with the dual mode cellular phone. According to D3 a mapping of IEEE 802.11 WLAN specific values are mapped to traditional cellular hand-over parameters for enabling seamless handover's between these heterogeneous networks. Throughout any call set-up processes, all communications between the dual mode cellular phone and the interworking node take place via a LAPDm (Link Access Protocol in the D channel-mobile) data link using previously assigned SAPI values during the registration process. All signaling messages (either GSM (Global System for Mobile Communications), TDMA (Time Division Multiplex Access), or CDMA (Code Division Multiplex Access)) exchanged between the dual mode cellular phone and the interworking node are formatted into LAPDm message frames. The LAPDm message frames are further encapsulated in IEEE 802.11 MAC (Medium Access Control) headers and transmitted across the WLAN air link. Although it is not specifically stated how paging is carried out in D3 it appears inherently disclosed that even paging messages are sent through the WLAN interface.
Prior art document “Interworking Between WLAN's and Third generation Cellular Data Networks”, A. K. Salkintzis, 2003, IEEE, pp 1803-1806 discloses a WLAN adaptation function (WAF) that supports interworking functions between GPRS (General Packet Data Radio System) signalling and data over 802.11 WLAN's. The system is intended for data services and no special means for voice services are defined.